The present invention relates to an aluminum or an aluminum alloy substrate for presensitized plates for use in making lithograhic printing plates (hereunder referred to as simply "PS plate(s)") and more particularly to a substrate for PS plates which sparingly cause contamination of non-image portions of the plate; which have high resistance to scratching thereof; and which are excellent in wear resistance, recovery from contamination with ink and printing durability due to improvement in the properties of the anodized film.
As light-sensitive plates for use in making lithographic printing plates, there have been known so-called PS plates composed of an aluminum plate and a thin film of a light-sensitive composition applied thereon. The aluminum plate is in general subjected to surface treatment such a mechanical technique as brush graining or ball graining; such an electrochemical technique as electrolytic graining; or a combination thereof to roughen the surface thereof, then etched with an aqueous solution of an acid or an alkali, anodized and thereafter optionally subjected to hydrophilization treatment to obtain a substrate for PS plates. Such a PS plate can be prepared by applying a light-sensitive layer thereto. The resultant PS plate is usually imagewise exposed to light, developed, retouched and gummed up to obtain lithographic printing plates which are then placed on a printing machine to perform printing operation.
However, non-image areas of a lithographic printing plate, which is produced by imagewise exposing a PS plate composed of a conventional substrate provided thereon with a positive-working light-sensitive layer and then developing the same, irreversively adsorb the substances present in the light-sensitive layer to cause contamination thereof which makes the differentiation .between the image areas and the non-image areas difficult during a retouching process and which leads to the formation of non-uniform plate surface because of the clear traces of retouching remaining thereon. In the worst case, such traces serve to cause contamination and, thus the plate is not applicable as a lithographic printing plate.
To eliminate such problems, for instance, Japanese Patent Un-examined Publication (hereinafter referred to as "J.P. KOKAI") No. 57-195697 proposes to additionally treat an anodized aluminum plate with a condensed sodium arylsulfonate. The contamination of the non-image areas can certainly be prevented according to this method, but on the contrary a new problem arises that the printing durability of the printing plate is reduced to 30 to 80% compared with that of the plate which has not subjected to the foregoing treatment.
On the other hand, Japanese Patent Publication for Opposition Purpose (hereunder referred to as "J.P. KOKOKU") No. 46-35685 proposes to treat the anodized aluminum plate with polyvinyl phosphonic acid. However, the aforesaid contamination of the non-image areas can only insufficiently be prevented when such a substrate is used.
When a conventional lithographic printing plate is used, background contamination of printed matters is caused since the ink attached to the non-image areas thereof is not rapidly removed. It can also be thought that the thickness of the anodized film may be increased to improve resistance to scracth of the non-image areas. However, in such a case, the contamination of the non-image areas becomes more and more severe and it cannot be eliminated by the aforementioned method for preventing the contamination.
As a method for preventing the contamination of the non-image areas by an electrochemical treatment, there has been known a method comprising subjecting an aluminum plate to barrier type anodization, forming a porous anodized film and then subjecting it to barrier type anodization (see J.P. KOKAI No. 53-2103).
Moreover, J.P. KOKAI No. 58-153699 discloses a method comprising forming a porous anodized film on the surface of an aluminum plate and then again anodizing it at a voltage of not more than 100 V in an oxo anion-containing electrolyte such as a boric acid solution.
However, in these methods, the barrier type anodization and re-anodization treatments are conducted at a voltage of not more than 100 V and these methods do not use a high voltage sufficient to seal pores of the porous anodized film. Therefore, the effect of preventing the contamination of the non-image areas is insufficient. In addition, if the thickness of the porous anodized film is relatively thin, the sealing of pores thereof can be achieved, but the resulting film is inferior in mechanical strength and the non-image areas are not resistant to scratching.
Moreover, it is found that if the grained surface is subjected to barrier type anodization, the printing durability of the resultant lithographic printing plate become low.